This invention relates to an amplifier system, and more particularly, to a system which is switchable between two amplifying operations.
Audio signal power amplifiers can be classified into a single-channel amplifier and a dual-channel amplifier. The single-channel amplifier amplifies a monaural signal, and the dual-channel amplifier is used to amplify a stereophonic signal.
Examples of these amplifiers are shown in FIGS. 1 and 2 in block diagrams. FIG. 1 is a conventional dual-channel amplifier and FIG. 2 is a BTL type single-channel amplifier which uses two amplifiers to produce a large output.
Referring to FIG. 1, a main part 100 of the single-channel amplifier is formed in a semiconductor integrated circuit which includes two feedback amplifiers 200 and 300 and two feedback resistors R.sub.f3 and R.sub.f4. Each of the feedback amplifiers 200 and 300 has an inverting input (-) and a non-inverting input (+). These amplifiers 200 and 300 are energized by a power supply source Vcc. A first input signal V.sub.S1 is applied to the non-inverting input (+) of the amplifier 200. A series connection of a resistor R.sub.f1 and a capacitor C1 is connected between the inverting input (-) of the amplifier 200 and ground to form a feedback circuit together with the feedback resistor R.sub.f3. An output terminal of the amplifier 200 is connected to a first load R.sub.L1 through a capacitor C.sub.3. Similarly, a second input signal V.sub.S2, a resistor R.sub.f2, capacitors C.sub.2 and C.sub.4 and a second load R.sub.L2 are connected to the amplifier 300.
The amplifiers 200 and 300 amplify respectively, the first and second input signals V.sub.S1 and V.sub.S2 and their amplification functions are independent of each other. This arrangement is preferable for amplifying a stereophonic signal.
Another type of amplifier is shown in FIG. 2 and is a BTL amplifier having a feature that a large voltage output can be obtained. In more detail, a main part 100 has a circuit configuration similar to the main part 100 of FIG. 1 with the difference residing in the circuit configuration outside the main part 100. An input signal V.sub.S3 is introduced to the non-inverting input (+) of the amplifier 200 and the resistors R.sub.f1 and R.sub.f3 and the capacitor C.sub.1 are connected to the amplifier 200 to form a feedback amplifier similar to FIG. 1. The output of the amplifier 200 is connected to one end of a load R.sub.L3 and to one end of a resistor R.sub.a5. A signal attenuated by a voltage divider composed of the resistor R.sub.a5 and a resistor R.sub.f2 is applied to the inverting input (-) of the amplifier 300 through a capacitor C.sub.2. The non-inverting input (+) of the amplifier 300 is grounded through a capacitor C.sub.5. A resistor R.sub.f4 is provided to operate the amplifier 300 as a feedback amplifier. An output of the amplifier 300 is connected to the other end of the load R.sub.L3.
With the BTL amplifier of FIG. 2, the output applied to one end of the load R.sub.L3 through the amplifier 200 has a reverse phase to that applied to the other end of the load R.sub.L3 through the amplifier 300. Therefore, the output obtained at the load R.sub.L3 is twice the output at the loads R.sub.L1 and R.sub.L2. This amplifier is preferable for producing a large output.
Referring to FIGS. 1 and 2, both amplifiers use the same main part 100. Thus, by changing the circuit outside the main part 100, it is possible to change the amplifier of FIG. 1 to the amplifier of FIG. 2 and vice versa. However, it is difficult in fact to assemble the amplifier so as to be switchable between either configuration. Further, there has been proposed no integrated circuit integrating two amplifiers with a switching circuit for changing the amplifier function. Because of this fact, a switchable amplifier using a conventional integrated circuit requires a number of circuit elements to be combined with the integrated circuit.